crypto/internal/cryptotest: x509-limbo test coverage

Augment existing test coverage by adding a harness for C2SP/x509-limbo.
This test suite is quite extensive, and it also subsumes the entirety of
the Netflix BetterTLS suite, allowing us to delete that harness.

x509-limbo publishes a JSON schema describing the test data. Similar to
Wycheproof, to avoid by-hand maintenance of corresponding Go structs,
while also avoiding a stdlib dependency on a generator, a separate Go
module is provided for generating the Go representation of the schema.
The resulting schema is vendored so it can be used without the
generator.

The upstream x509-limbo project has been running the test corpus against
the Go standard library x509 package for some time (using Go 1.25.5). To
get started we've allowed the unexpected verifications and unexpected
failures based on the upstream results that predate running the suite
ourselves. In some cases we will want to fix the behavior on our side
and remove these allows, in other cases we have chosen to diverge from
the upstream expectations and will leave the allow with a justifying
comment. This will let us catch new regressions while also allowing
landing the test without fixing these findings up-front.

Change-Id: Ib0d3876da7daf864571b6b0924a36e46bf446d45
Reviewed-on: https://go-review.googlesource.com/c/go/+/728320
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Auto-Submit: Daniel McCarney <daniel@binaryparadox.net>
TryBot-Bypass: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Neal Patel <nealpatel@google.com>